Wireless local access network (WLAN) standards based on an orthogonal frequency division multiplexing technology (OFDM) include gradually evolved releases such as 802.11a, 802.11n, and 802.11ac. 802.11n and 802.11ac already support single user multiple-input multiple-output (SU-MIMO), and 802.11ac further supports downlink multi-user multiple-input multiple-output (MU-MIMO). Currently, the IEEE 802.11 standard organization has started standardization work of a new generation WLAN standard 802.11ax, which is referred to as a high efficiency WLAN (HEW for short). Orthogonal frequency division multiple access (OFDMA) and uplink MU-MIMO are two key technologies in 802.11ax. In SU-MIMO and MU-MIMO, multiple spatial flows are transmitted in parallel by means of MIMO; before performing receiving processing, such as demodulation, on all spatial flows, a receive end first needs to obtain a MIMO channel estimate. For example, in uplink MU-MIMO, to demodulate signals from different stations (Station, STA for short), an access point (AP) may obtain, by using high efficiency (HE) long training fields (LTF) (HE-LTF for short) in preambles of uplink packets sent by all STAs, a channel estimate of uplink MU-MIMO.
FIG. 1 is a schematic diagram of an existing HE-LTF solution. In the existing solution, an HE-LTF includes N OFDM symbols, where N is a quantity of spatial flows (when a quantity of actual spatial flows is an odd number greater than 1, N is the quantity of the actual spatial flows plus 1; when a quantity of actual spatial flows is 1, N=1). Each row in FIG. 1 represents a subcarrier and each column represents an OFDM symbol. Each icon (e.g., ●, , ◯ and ▪) represents a different reference signal (RS) pattern. For example, in the first row (i.e., first subcarrier), the first OFDM symbol carries RS pattern ●, the second OFDM symbol carries RS pattern , the third OFDM symbol carries RS pattern ◯, and the forth OFDM symbol carries the RS pattern ▪. For the same subcarrier, the OFDM symbols are divided into groups each of which includes four OFDM symbols in this example. In each group, the first OFDM symbol always carries RS pattern ●, the second OFDM symbol always carries RS pattern , the third OFDM symbol always carries RS pattern ◯ and the forth OFDM symbol always carries RS pattern .
Available subcarriers of each OFDM symbol carry reference signals, and sequentially in time correspond to different spatial flows by means of subcarrier interleaving. An available subcarrier is a subcarrier except if the subcarrier is a zero-frequency subcarrier or a protection subcarrier that is used to suppress adjacent channel leakage of an MIMO transmission frequency band. Specifically, a quantity of subcarriers corresponding to each spatial flow in an OFDM symbol is M/N, where M is a quantity of available subcarriers. All the spatial flows sequentially correspond to different subcarriers in each OFDM symbol, and subcarriers that are corresponding to a corresponding spatial flow and that are in every two OFDM symbols are staggered by a location of one subcarrier. Therefore, within a range of N OFDM symbols, subcarriers corresponding to each spatial flow are distributed to locations of all available subcarriers, and the subcarriers corresponding to all spatial flows are mutually orthogonal. In this way, a channel estimate, of a corresponding spatial flow, on each available subcarrier is obtained by using reference signals carried by subcarriers corresponding to each spatial flow in the HE-LTF.
For example, and with reference to spatial flow 1 in FIG. 1, the first OFDM symbol (LTF-1), RS pattern ● which corresponds to spatial flow 1 occupies 1st, 5th, 9th . . . subcarriers. In the second OFDM symbol (LTF-2), RS pattern ● which corresponds to spatial flow 1 occupies 2nd, 6th, 10th . . . subcarriers. Therefore, subcarriers which carries RS pattern ● in one OFDM symbol are different from those in other OFDM symbols. And in consecutive OFDM symbols, the sequential number of the subcarriers that carry RS pattern ● is added by one, for example, 1st, 5th, 9th in former OFDM symbol and 2nd (1st+1), 6th (5th+1), 10th (9th+1) in letter OFDM symbol. When the OFDM symbols continuously goes on, the subcarrier that carries a given RS pattern will traverse from the 1st subcarrier to the last subcarrier.
For ease of description, in the present invention, a distribution pattern of locations of subcarriers corresponding to all spatial flows in an nth OFDM symbol of the HE-LTF is defined as Ψ(n). Locations of subcarriers corresponding to different spatial flows are distinguished by using different symbols, and a distribution pattern of locations of subcarriers corresponding to all spatial flows in a next OFDM symbol may be represented as Ψ(n+1). “+1” indicates that locations of subcarriers corresponding to all spatial flows in the OFDM symbol are moved forward or backward by a location of one subcarrier. In this way, in the existing HE-LTF solution shown in FIG. 1, distribution patterns of locations of subcarriers corresponding to all spatial flows in N OFDM symbols are sequentially Ψ(1), Ψ(2), . . . , Ψ(N).
An existing WLAN system based on the OFDM technology uses an OFDM symbol whose length is 4 us. To support outdoor application and improve OFDMA performance, the 802.11ax standard supports the use of an OFDM symbol whose length is four times the length of an existing OFDM symbol or greater. When an OFDM symbol whose length is four times the length of an existing OFDM symbol is used, it indicates that a length of each OFDM symbol is 16 us. For example, for a typical WLAN packet whose length is 1-3 ms, when eight spatial flows are transmitted, a length of an HE-LTF is up to 128 us, an overhead is up to 4.3% to 12.8%, and consequently resource utilization is low.